FIG. 9 shows a conventional method of controlling arc welding. Welding voltage Vw, welding current Iw, speed setting signal Fr for feeding a wire, and feeding speed Fs of a tip of the wire are shown in the figure, as they change with the time respectively. A short-circuiting period Ts between t1 and t2 is shorter than predetermined time period Tt, and signal Fr remains at a level of Ffr for setting forward feeding of the wire, which hence maintains feeding of the wire at a speed of Ffs. The wire is fed continuously at the same speed Ffs in the succeeding arcing period Ta between t2 and t3.
Signal Fr still remains at the level of Ffr in a short-circuiting period commencing from time t3, and the wire is fed at the speed of Ffs until time t4 after an elapse of time Tt from the time t3. At time t4, signal Fr changes to another level Frr for setting withdrawal of the wire, and wire feeding speed Fs begins to decrease at a slope determined by an inertia. At the same time, welding current Iw is reduced to a low level.
Wire feeding speed Fs decreases to below zero, that is, in a reverse movement. At time t5, the wire tip separates from a base metal to break the short circuit, and arcing restarts again. At the same time, signal Fr is switched to the level Ffr, and wire feeding speed Fs starts to increase toward the speed Ffs. Welding current Iw reaches a value corresponding to wire feeding speed Ffs as determined by a constant-voltage characteristic of welding voltage Vw and an arc load.
In the conventional method of controlling arc welding described above, there is a problem in arcing stability because of the presence of the normal short-circuiting period from time t1 to t2 and another short-circuiting period from time t3 to t5, which is much longer than the normal period. Japanese Patent Unexamined Publication, No. 2004-298924, is an example that discloses the above-described conventional method of controlling arc welding.